Add instrumentation to the search services

[renchap]

This is a start at adding custom spans to the app code. I started on this one as I noticed some weird traces when doing searches, and I wanted to have a better visibility on which kind of search was triggering this.

We need the opentelemetry-api to be loaded so we can call the various instrumentations methods, but if OTEL is not configured then those methods are no-ops.

I created a custom mastodon tracer. At some point we may want to have multiple tracers to split the various parts of the app into their own instrumentations, but this can be done later once we have a better vision of what we want.

cc @robbkidd